Skylights add natural light to building interiors
Bird's Eye View
Skylights brighten rooms even on overcast days
Skylights serve many purposes. They brighten interior spaces, reducing the need for artificial light. Skylights that open and close introduce fresh air as they vent stale air that collects at the top of the house. Some skylights are big enough to serve as emergency exits, and skylights can open up panoramic views in lofts and attics.
Skylights are sometimes called roof windows. The distinction may seem minor, but in the industry, “skylights” are out of reach while “roof windows” are within reach by occupants, so they need to operate more or less like any other window. Skylights and roof windows can be installed on sloped or flat roofs, and are compatible with virtually any type of roofing material. Factory-supplied flashing kits and flashing integral to skylight frames have dramatically reduced complaints of water leaks.
For rooms some distance away from a roof, a type of skylight called a tubular skylightRound skylight that transmits sunlight down through a tube with internally reflective walls, even through an attic space; it delivers daylighting through a ceiling light diffuser. Most tubular skylights are 12 to 16 inches in diameter and deliver daytime lighting comparable to several 100-watt incandescent light bulbs. or light tunnel can route natural light from a roof collector to rooms 20 feet or more away. While not skylights in the traditional sense of the word, they provide ample natural light to spaces that would otherwise be dark and gloomy.
Other than the potential for leaks, the major drawback to skylights is an energy issue: overheating during the summer and energy losses during the winter. Newer types of glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill., as well as accessories such as built-in sun shades, can help minimize the problem.
Glazing choices are many
Skylights are manufactured with either plastic or glass glazing. Skylight frames can be made of wood, aluminum, or polyvinyl chloride (PVC).
Manufacturers have made huge strides in broadening the variety of glass that's available in skylights. Double-glazed skylights with low-eLow-emissivity coating. Very thin metallic coating on glass or plastic window glazing that permits most of the sun’s short-wave (light) radiation to enter, while blocking up to 90% of the long-wave (heat) radiation. Low-e coatings boost a window’s R-value and reduce its U-factor. glass are now common. But skylights with three and even four panes of glass also are available, further reducing heat transfer in both winter and summer. Laminated, tempered and hurricane-resistant glazing is available, as well as "electrochromic" glass that changes opacity by means of an electric charge.
Plastic glazing is typically less expensive than glass, and it can be molded into domes and other shapes that are difficult if not impossible to achieve with glass. At the same time, plastic also is softer, easier to scratch and may yellow over time.
Choose the right skylight for the situation
Heat gains and losses can be a big problem with skylights. In cold climates, heat gains during the winter are a plus, helping to reduce heating costs just like other passive solar features. But in the summer, you'll not only feel like a chicken in a Bombay tandoor, but you'll see higher cooling costs as well. The California Energy Commission says that a 2-ft. by 4-ft. skylight with single glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. will increase the energy required for cooling by 240 kilowatt-hours per year. Better glazing, of course, reduces the effect.
A skylight on a north-facing room won't contribute as much heat to a room as a south- or west-facing skylight. A skylight on an east-facing room gives the most heat gain and light in the morning. South-facing skylights are best for passive solar heating in the winter, the Energy Commission points out, but also produce the most heat during the summer. Designers can orient skylights accordingly.
The other major variable is the type of glass (or plastic) used in the skylight. By choosing glazing with specific values for Visible Transmittance, U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. , and Solar Heat Gain Coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1., designers can tune skylights for a specific use or location, just as they would for windows. Looking up the values for a particular skylight is as easy as going to the website of the National Fenestration Rating Council and following the link to the Certified Product Directory.
Keeping the water where it belongs
Residential skylights are made in two basic versions: deck-mounted and curb-mounted. Deck-mounted units, which are attached directly to the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , are designed to work at a specific roof pitch, usually 3-in-12 or greater. For roofs with a lower pitch, a site-built curb made from dimensional lumber can be added to the rough opening.
The other factor that limits water intrusion is careful flashing. Manufacturers now offer their own flashing kits for their skylights; some flashing kits are even shipped in the same box as the skylight to eliminate any mix-ups on the building site. When used with peel-and-stick membranes and other materials as directed by the manufacturer, builders shouldn't be troubled by callbacks for leaks.
Velux offers a 10-year guarantee and says its skylights (when properly installed) "simply don't leak." Wasco's residential skylights are made with a co-extruded PVC frame that incorporates a wide waterproofing flange.
Interior condensation is another concern with skylights, but some manufacturers have taken steps to control that, too. GlazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill. with lower U-factors (the same as higher R-values) helps, and some units are designed to collect any condensate at the edge of the glass and funnel it outside or collect it so it can evaporate.
Section 2405 of the 2012 International Building Code lays out a variety of requirements for skylights, covering impact and wind resistance and maximum U-values and Solar Heat GainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. Coefficients for different regions of the country. There also are specific requirements for skylights used for emergency egress, and separate provisions for the percentage of roof area that can be occupied by skylights in commercial buildings. Some local jurisdictions may have their own requirements.
Ventilation, views and light
Skylights come in many forms, from inexpensive plastic domes to operable roof windows that double as rooftop emergency exits. Even the most basic skylight admits daylight into interior spaces that otherwise would have to be illuminated electrically. Skylights that open and close have the added advantage of venting stuffy air from upper story rooms. Depending on how they are placed, skylights also can provide great outdoor views.
Both plastic and various kinds of glass are used for skylight glazingWhen referring to windows or doors, the transparent or translucent layer that transmits light. High-performance glazing may include multiple layers of glass or plastic, low-e coatings, and low-conductivity gas fill.. Plastic, usually acrylic or polycarbonate, costs less than glass, but it's not as resistant to scratches and after long exposure to sunlight it may become brittle and discolored. Polycarbonate has higher impact resistance and is more expensive than acrylic. Plastic skylights are manufactured in a variety of shapes — domes, rectangles, circles, ovals, and triangles — that would be difficult to duplicate with glass. More expensive skylights are made with glass, and manufacturers now offer a tremendous variety of glazing options. That can be useful in choosing skylights for a particular roof location, or for a particular climate zone.
Skylights are manufactured by many companies, although there are only a few national players. The residential market for off-the-shelf skylights is dominated by Velux, a Danish company. Maine-based Wasco claims a distant second place.
Other manufacturers, such as Tam, Columbia,and Sun-Tek, are stronger regional forces. Fakro, is a major European manufacturer but it hasn't made serious inroads in the U.S. to date. Three major window manufacturers ( Andersen, Marvin and Pella) have dropped out of the skylight business in recent years as the industry consolidated.
In addition to mass-marketed residential skylights, there's also a booming trade in high-end custom skylights for residential as well as commercial buildings. Wasco considers itself the leader in that field.
Matching skylights to the roof slope
Skylights can be mounted directly to the roof deck or attached to a manufactured or site-built curb that keeps the unit above the plane of the roof and is set at the correct angle. Manufacturers specify a minimum roof pitch for deck-mounted skylights to ensure they shed moisture correctly. A minimum roof slope of 15 degrees, the same as a 3-in-12 pitch, appears typical.
The installation angle, determined either by roof pitch or a separate curb, also affects now much heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. the skylight will contribute to the building, a major energy consideration. A skylight on a low-slope roof will let in more heat in the summer when the sun is high in the sky and less in the winter, when the sun is closer to the horizon, an effect that will be more pronounced in northern climates. As the U.S. Department of Energy points out, that's exactly the opposite effect most people want to achieve. The department recommends that to make the best use of solar gain, the skylight should be installed at a slope equal to the site's latitude plus 5 to 15 degrees. In San Francisco, with a latitude of 35 degrees north, a south-facing skylight should be sloped at between 40 and 50 degrees. That would minimize the amount of solar gain during the summer, and reap the best energy rewards in winter.
Skylights can be installed in both vaulted and flat ceilings. For installations over a flat ceiling, a light well is constructed between ceiling and roof. The well can be built in various ways to spread the light, but the top usually is perpendicular to the roof plane and the bottom of the well is plumb. The sides of the well are typically plumb. Unless the attic is a conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. , the light well should be insulated and air-sealed carefully.
MORE ABOUT SKYLIGHTS
Living with energy shortcomings
For all their benefits, skylights also have an Achilles’ heel — their negative impact on energy efficiency. Skylights are, after all, thinly disguised holes in the roof. In a high-performance house, let's say with a roof insulated to R-60, even a modestly sized skylight is a breach in the building's thermal envelope.
Unwanted heat gainIncrease in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss. in summer is one issue, but so is the loss of heat in winter. According to the California Energy Commission, a skylight will typically lose 35% to 45% more heat during cold weather than a window of the same size installed vertically.
"That's because heated air rises," the commission says in an online report. "As the warm air in a house rises toward the ceiling, it comes in contact with the cold surface of the skylight. The air is cooled and falls, forming a large draft loop that can dramatically decrease comfort. The more nearly horizontal a skylight is, the more significant the effect is." This convection loop takes place even in the space between panes of glass in double-glazed skylights, and is more pronounced when the space between panes is more than 1/4 in.
In addition to convection, the commission says, skylights also lose heat through radiation more dramatically than windows installed vertically. Losses are magnified when the skylight is installed on top of a curb, which increases the total surface area exposed to cold night air.
Because skylights face the sky so directly, they can let in substantially more heat than a vertical window of the same size. While that's an advantage during heating season, it's offset by decreased comfort and higher cooling loads during the summer.
"I wouldn't put skylights in primarily to gather solar energy for heating," says Marc Rosenbaum, the director of engineering for the South Mountain Company, "because they'll still gather more in the summer than in the winter."
Glazing can minimize energy impact
Just as window manufacturers have steadily improved the performance of glass with multiple pane designs, low-emissivityAmount of heat radiation emitted from a particular body or material. Emissivity is expressed in a fraction or ratio, with the lowest values indicating low emissivity and the highest indicating the high emissivity of flat black surfaces. coatings, and inert gas fills, skylight makers also are expanding their offerings. Wasco makes several triple-glazed windows. So does Fakro, which also has started producing a skylight with four panes of glass. This unit, says Fakro's U.S. manager Waldemar Szalus, has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 10, equating to a U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. of 0.1, which Fakro says is the best in the industry. (Keep in mind that European and North American windows are tested with different protocols, so U-factors aren't directly comparable.)
At least for now, many other manufacturers seem to be sticking to dual-paned glazing. Among them is Velux, which offers a triple-pane skylight in the U.K. but not in products sold in the United States.
"Because," says Ross Vandermark, the U.S. product manager for the company, "our entry-level glass, so to speak, is a low-e 3, argon-gas fill. It's a very, very energy-efficient pane, so we kind of start you out at a high level. You're getting a very good U-factor, a very good solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1., because we're not starting off with clear class or a single low-e coatingVery thin metallic coating on glass or plastic window glazing that reduces heat loss through the window; the coating emits less radiant energy (heat radiation), which makes it, in effect, reflective to that heat; boosts a window’s R-value and reduces its U-factor. ."
The thermal performance difference between a double-glazed window with a U-factor of 0.37 and a four-pane assembly with a U-factor of 0.1 is roughly the same as comparing an R-2.7 wall with an R-10 wall. One blocks the flow of heat nearly four times as effectively as the other.
But the difference that makes for a relatively few skylights isn't nearly as much as it would be for a house's worth of windows, which may explain why double-glazed units are by far and away the most common.
"If you do a building model and you figure the difference in your heating bill or your cooling bill with triple-glazed vs. double-glazed, it's like $11 a year that you save," says Wasco CEO Jeff Frank. "If you pay a $250 or $300 premium for that, your payback is so far out there. Unlike windows, where you're going to have 15 or 20 times the square foot area of openings that you are with skylights in a home, in that kind of application you may even be able to downsize your furnace system, so the impact can be hundreds of dollars a year. If you're cutting a hole in your R-30 roof, does it matter if it's an R-value of 3.2 or 5? From the standpoint of energy, it doesn't make a lot of difference."
What savvy designers look for is the best combination of U-factor, solar heat gain, and visual light transmittance for a specific installation. And the best source of that information for skylights, as it is with windows, is the National Fenestration Rating Council, which maintains a data base searchable by manufacturer or by the type of skylight.
Other variations in glazing and accessories
There are many other options in skylight glazing, including:
- Electronically tintable glazing, also called "electrochromic" glass, made by SageGlass. It becomes darker when an electric current passes through thin ceramic layers on the glass, a process that can be activated manually or automatically. The glass can block virtually all solar heat gain (with the SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as
a number between 0 and 1. falling to 0 .04) but can be returned to a clear state to let in heat and light as needed. It's useful in maintaining even light levels inside, opening up when a cloud passes overhead, for example, and darkening when there's no cloud cover. SageGlass says is takes no more than 60 watts of electricity to control 2,000 sq. ft. of glass. Electrochromic glass has been prohibitively expensive, some $120 a sq. ft. as little as three years ago, says Wasco's Jeff Frank, but it's now in the $50-a-sq.-ft. range, and prices should fall to $20 or $25 in time. That's still very expensive residential glass, but it can actually be cost-effective in commercial applications when AC systems can be downsized and mechanical shades eliminated.
- Out-of-reach skylights that can be opened and closed electrically. Some close automatically at the first sign of rain.
- Laminated and tempered glass for those installations required by code, as well as hurricane-resistant glass in high-risk coastal areas such as Florida's Dade County. In areas where deep snow is a concern, you can buy special snow-load glass.
- Shades and blinds. A variety of moveable shades and blinds can block or filter light as needed. Velux offers a solar-powered (photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow.) operated version, and says its black-out blinds improve energy performance by 45%.
- Roof windows that hinge on the side, like a door, that can be used as an emergency exit from an attic, as well as windows that hinge at the bottom to ventilate smoke from the building during a fire (both of these by Fakro).
- A type of translucent insulation called Lumira Aerogel by Cabot Corp made almost entirely of air and said to be the lightest solid ever produced. The material has an R-value of 8 per inch. Skylights incorporating aerogel give off a diffuse, non-glaring light while limiting heat losses in winter. It currently has wider applications in commercial buildings than in residential products, probably because of the high cost of producing it, but it can make a huge energy difference for large commercial buildings when daylighting and AC costs are factored in.
- Wasco Skylights
- Scott Gibson
Apr 8, 2013 10:32 AM ET